Over fifty years ago it was recognized that synthetic butyl rubber was a quite effective substitute to natural rubber in many respects. Because of its low price, its temperature stability, and its effectiveness in a wide range of rubber formulations, butyl rubber rapidly displaced natural rubber in automobile tire applications and became widely used in household and industrial products. Today, butyl rubber, polyisobutylene, and other polymers of isobutene are produced in a wide range of high-molecular weight, elastomeric grades.
Nearly thirty years ago it became apparent that isobutene made by decomposing alkyl tertiary-butyl ethers met the purity requirements of commercial polymerization-grade isobutene. However, these alkyl tertiary-butyl ethers remained in short supply until the last decade when, as environmental pressures over anti-knock additives in gasoline increased, production of these alkyl tertiary-butyl ethers by etherifying mixed butenes boomed, and a concomitant search for a decomposition process followed. Two of the chief criteria for a decomposition process are yield and impurity.
Yield is defined as the percentage of total alkyl tertiary-butyl ether in the feed which appears as isobutene in the product and can be expressed by the equation: EQU Y=P/F*100
where Y equals percent yield, P equals moles of isobutene in the product, and F equals moles of alkyl tertiary-butyl ether in the feed. The higher the yield, the more desirable is the process.
High purity is a major requirement of isobutene feedstocks used for polymerization. This was also the case when butyl rubber was first produced, because of requirements on molecular weight and heat and chemical resistance. Today's high-molecular weight grades of butyl rubber and polyisobutylene require a commercial polymerization-grade isobutene of &gt;99.5 wt% purity with impurity limits for isobutane, propylene, pentenes, oxygenated compounds, and water, but as used herein the term "impurity" is intended to indicate a concentration of total normal butenes in parts per million by weight in the isobutene product.
Consequently, the ideal process is one where the yield equals 100 and the impurity equals zero. The minimum requirement is that yield be at least 92% and impurity be not more than 500 ppm by weight. These are minimum requirements; that is, if a process fails to meet these requirements simultaneously the process is commercially unacceptable. The impurity requirement is assuming added importance and significance in view of the expectation in some areas of minimum standards for impurity in isobutene of not more than 300 ppm by weight near-term.
The isolation of isobutene from mixtures of C.sub.4 hydrocarbons by the combined process of etherifying isobutene and subsequently decomposing methyl tertiary-butyl ether (MTBE) is well known and described in the paper authored by Fritz Obenaus et al. entitled "Huels Process: Methyl Tertiary Butylether", presented at the AIChE 85th National Meeting in Philadelphia, Jun. 4-8, 1978 and in the article starting at page 109 of the December 1979 issue of "Hydrocarbon Processing." The individual processes of olefin etherification and ether decomposition for C.sub.4 hydrocarbons are described in "Ullmann's Encyclopedia of Industrial Chemistry," Volumes A4 and A16, Fifth Edition (VCH, Weinheim, Germany).
Flow schemes for the combined process of olefin etherification and ether decomposition with means to decrease the contaminants in the isobutene product are well known. U.S. Pat. No. 4,570,026 and the article starting at page 101 of the August 1981 issue of "Hydrocarbon Processing" describe recycling by-products with unreacted MTBE from a decomposition section to an etherification section, and ultimately bleeding out the by-products from upstream of the decomposition section in an intermediate product stream of MTBE. Where there is no intermediate product stream of MTBE, the August 1981 "Hydrocarbon Processing" reference suggests removing the by-products using a "recycle purification unit" on the MTBE recycle stream between the decomposition section and the etherification section; however, what the "recycle purification unit" comprises is not described. U.S. Pat. No. 4,287,379 and U.S. Pat. No. 4,320,232 teach purging out the by-products via an alcohol-containing stream from either the side of a distillation column downstream of the decomposition reactor or from the alcohol recycle stream between the decomposition section and the etherification section.